It's his signature... As lisa says, his signature has changed over the years. It does mean that he signed this in the last 10 years or so and not immediately after the photo was taken... which ought to lower the value slightly.

David did go through a phase where - if he signed items which were older - he would add the year in which he signed it.

Greetings from the Dutch mountainsSven

I recognize no method of living that I know of, I see only the basic materials I may use